Deepwater Sequence Stratigraphy and Reservoir Prediction

By

Stephen S. Flint¹, Peter Sixsmith¹, Martin Grecula¹, Graham Potts¹, Stephen Johnson²

(1) University of Liverpool, Liverpool, United Kingdom (2) Statoil, Forus, Stavanger, Norway

 The early Karoo basin of southwestern South Africa was segmented into the Tanqua and Laingsburg sub-basins through the growth of antiform/synform pairs oblique to the dominant shortening direction in the bounding Cape Fold Belt. These structures grew episodically during deposition in the Laingsburg area but did not affect the Tanqua sub-basin. Detailed sedimentological analysis and regional 3-D mapping of basin floor and slope turbidite complexes from both sub-basins has allowed development of a high resolution sequence stratigraphic model with modifications to account for the influence of episodically growing basin floor topography in the Laingsburg sub-basin. Individual basin floor fans (50-300 m thick) can be divided into high frequency sequences that include sandy growth phases (lowstand systems tracts) separated by fan-wide zones of reduced sand deposition (transgressive and highstand systems tracts). In both sub-basins, these high frequency sequences form zones for reservoir modelling and stack in progradational, aggradational and retrogradational styles, which controls vertical and horizontal effective permeability. The main effect of punctuated growth folding on the Laingsburg basin floor fans was to elongate fan geometries parallel to the fold axes and to concentrate sand-rich flow fractions in syntectonic lows, resulting in starvation on and beyond highs. Saddles in the antiforms allowed cross-structure sand transport. The main effect of growth folding on slope systems is episodic cycling (in space and time) between ponded accommodation and bypass, resulting in complex facies patterns, net:gross trends and connectivity.